The present invention relates generally to photocurable dental compositions and more specifically to a device provided with an axial array of light emitting diodes directed along a plurality of divergent paths such that a position insensitive optical field is generated.
Certain polymeric materials useful in the field of dentistry for adhesion, sealing and restoration may be cured or hardened upon exposure to a source of radiation. Such photoactive materials are known as xe2x80x9cphoto-curable dental compositionsxe2x80x9d and generally harden when exposed to radiation having wavelengths in the visible range. Photo-cured dental compositions are convenient for use by a dentist because the curing process can be initiated when the dental composition has been accurately placed in its proper position. A source of radiation energy positioned proximate to the material to be hardened, for example an appropriate amount of composition placed inside a tooth cavity, is activated to initiate polymerization and subsequent curing of the composition to secure the repair. Early methods for curing photosensitive dental compositions included dental guns and other apparatuses for producing concentrated beams of UV radiation. See U.S. Pat. Nos. 4,112,335 and 4,229,658, for example. Later, visible light curable dental compositions were used and dental radiation guns for producing concentrated visible light were provided like that disclosed in U.S. Pat. Nos. 4,385,344 and 6,171,105. However, a relatively high divergence about 25 degrees of the light beam from such visible light sources reduces penetration into the tooth structure, leading to their relative inefficiency and unreliability for photo-curing dental composition that are thicker than about two millimeters.
Photo-curable dental materials have also been developed that are hardened by exposure to radiant energy in a pre-selected spectral range. Typically, a photo-activated chemical reaction in many photo-curable dental materials is initiated by application of a high intensity blue light having a wavelength of 400-500 nanometers. Since the light sources employed typically produce the entire visible light spectrum as well as some non-visible radiation, a reflector is coated to reflect only visible light, and the filters are selected to substantially block non-visible radiation and visible light other than blue light in the range of 400-500 nanometers, in order to produce the desired range of radiation, as shown for example in U.S. Pat. No. 5,147,204. Other high power arc sources, such as the one disclosed in U.S. Pat. No. 5,879,159, produce filtered wavelengths in the 430-505 nanometer range. Laser based radiation sources have also been employed, using for example, an argon-ion laser producing either specific wavelengths or their combinations in the 450-514 nanometer range. See U.S. Pat. No. 5,616,141. U.S. Pat. No. 6,099,520 discloses a portable, cordless, hand-held device that uses a diode-pumped microchip laser emitting radiation at 480 nm.
There are several disadvantages in using light curing apparatuses of the prior art like those discussed above. Commercially available dental light guns that use metal halide or plasma ark lamps often include an elongated, slender light guide such as a bundle of optical fibers having a free end that can be positioned close to the photo-curable material in order to direct light to the material from a light source located outside the oral cavity. The bundle of optical fibers is an added component that reduces the efficiency of the light reaching the curing site. Thus, because of the relatively large size of the dental gun within a patient""s mouth, a degree of physical discomfort is introduced to the patient as well as to the dentist who must hold the gun steady for about one minute. These sources produce all visible and some non-visible wavelengths and use band pass filters to admit wavelengths of interest. The result is a heating of the device that must be cooled using a cooling fan or other means.
Second, the area illuminated by conventional blue-filtered metal-halide radiation is usually in the range of about a xc2xd-inch diameter circle and over a typical curing cycle of about 60 seconds. The relatively high energy output and beam divergence of such dental guns leads to the possibility of increased heating of the pulp tissue which is sensitive to small changes in temperature.
The argon laser sources are bulky and transport of laser light from the argon laser source to the curing site can only be accomplished by a long fiber-optic delivery system. The technology of either the argon laser or the diode-pumped microchip laser is complex and prevents inexpensive implementation. Their maintenance and repairs are also expensive Lasers are intrinsically inefficient devices meaning that a very small portion of the electrical energy is finally converted to useful light. Furthermore there is the danger of accidental exposure of coherent laser radiation to the eye of either the dentist or the patient during the dental procedure resulting in a damage that could be greater than that resulting from incoherent radiation.
In addition, when dental compositions are cured in place within a cavity for instance, after curing an amount of shrinkage of about 2.5-3.0% occurs leaving a gap within the area being treated; such shrinkage is so deleterious that any small reduction in shrinkage is desirable.
Furthermore, in tests of cure depth uniformity of standardized compositions, it was found that a high percentage (46%) of curing lights used in private dental offices are unsuitable for use when tested against manufacture""s recommendations using a curing radiometer or a heat radiometer, due in part to the loss of output of the light source in use [J Dent Mar. 27, 1999 (3):235-41]. Finally, due to the expenses of combining a laser or metal-halide radiation source, focusing elements, power sources, etc., significant expense are involved in purchasing and using dental guns. Conventional dental curing devices are therefore seen to have shortcomings including uncomfortable use, unreliable curing and relatively high expense.
U.S. Pat. No. 4,385,344 discloses a dental gun device for production of light in the low visible range for photo-curing dental compositions, the device comprising a tungsten halogen lamp with a concentrating reflector, which reflects visible light and passes middle and far infrared wavelengths. A dichroic heat reflecting filter which passes light from 400 to 700 nm and reflects energy in the visible red and near infrared wavelengths back to the lamp envelope, enhances lamp halogen cycle efficiency. The dichroic heat-reflecting filter is followed by a dielectric filter, which provides a high efficiency bandpass at the desired visible range. A fiber optic light guide is positioned to receive the focused and filtered light and to transmit it to a reduced surface light-applying tip at the end of the handpiece. The fiber light guide is encased in a specially designed sheathing, which provides protection to the optical fibers and carries two electrical conductors which are connected between a control switch on the handpiece and the power supply for the lamp.
U.S. Pat. No. 5,147,204 is representative of conventional blue-light filtered dental guns. This patent discloses a blue light emitting apparatus for curing photo-curable dental material including a handpick having a housing, a depending handle and a detachable light guide. The light guide is received in a head connected to the housing. A source of tungsten-halogen light is coupled to the housing, and a light guide is detachably connected to the head for communication with the source of light. Since the tungsten-halogen light produces the entire visible light spectrum as well as some non-visible radiation, a reflector is coated to generally reflect only visible light, and a blue-pass filter and a heat filter are selected to substantially block non-visible radiation and visible light other than blue light in the range of 400-500 nanometers.
Still further devices and techniques have been proposed as noted below.
There is shown in U.S. Pat. No. 5,420,768 to Kennedy, for instance, a portable photo-curing device that has a light emitting diode matrix which is energized with battery power. The ""768 patent notes in Column 2 that LEDs of various selected colors may be formed on the module by using selected color dyes so that the emitted light is a pure white light or a combination of selected color lights to provide a predetermined photo curing effect. The light emitted by the LEDs may have a peak wavelength of 470 nm which is used for photo curing purposes.
U.S. Pat. No. 5,634,711 to Kennedy et al. discloses a portable light emitting device suitable for medical and industrial photo curing. It is noted in the ""711 patent that various applications require different light dosage values. For example, it is noted in Column 1, lines 39 and following that light dosage values in the range of up to 400 mW/cm2 are typically required for dental applications. On the other hand, a medical application such as photodynamic therapy of psoriasis and basal cells requires much lower power typically in the range of up to 100 mW/cm2. The device according to the ""711 patent includes generally a power supply, a housing, and a substrate upon which a plurality of light emitting diodes are mounted. It can be seen from FIGS. 1 and 6 of the patent that the LED array is generally planar and that the device typically includes an optical assembly such as a fiber optic taper. Here again, the LED can comprise xe2x80x9cbluexe2x80x9d LEDs with a spectral emission in the 470 nanometer range. Typically, the LEDs are driven by a pulsed power supply in order to minimize heat generation.
U.S. Pat. No. 5,711,655 to Adam et al. discloses a method and apparatus for bonding orthodontic brackets to teeth. The subject device includes a base with a central opening and a body with a passage aligned with that opening. A curing light assembly includes an outer end portion that is removably received in the passage for curing adhesive beneath the central section of the bracket body. Once the adhesive beneath the central section of the bracket body is cured to temporarily tack the bracket base to the patient""s tooth adhesive extruded from the peripheral edge of the bracket base can be readily removed without dislodging the bracket from its intended position. The device can be used to create a temporary xe2x80x9ctackxe2x80x9d bond while another device is used to fully cure the composition. See Col. 8-9. A curing light assembly optionally includes a focusing lens for an LED emitter which may be a dome shaped lens that covers the diode. See Column 6, lines 45 and following.
U.S. Pat. No. 6,102,696 to Osterwalder et al. discloses a self-contained light source for curing light initiated resins used to coat teeth as veneers and fill cavities and chips in teeth in aesthetic or restorative procedures. The source includes an elongated container holding a battery and an electronic compartment in one end and a light emitting window at the other. A plurality of closely spaced light emitters, typically light emitting diodes or laser diodes are arrayed in a radial or arcuate configuration to direct light to a common focal point. The light is directed out of the container toward a tooth bearing the resin to be cured to a hard stable state. The light emitters produce light in a region of the spectrum to which the resin curing initiator is sensitive, typically blue light. It can be seen from FIGS. 2 and 3 of the ""696 patent that LEDs are typically arrayed in an arcuate configuration about a focal point 38. The apparatus is reported to be useful for curing dental resins including a 1:1 mixture by weight of bis-phenol-2 bis (2-hydroxypropyl) methacrylate and tri (ethylene glycol) dimethacrylate monomers. The resinous mixture may further include a camphorquinone photoinitiator and a tertiary amine reducing agent. Fillers such as silica particles and colorants are typically included to achieve the desired hardness level and color.
U.S. Pat. No. 6,159,005 to Harold et al. discloses an apparatus for photopolymerizing synthetic materials, specifically dental materials containing camphorquinone or phosphine oxide as photoinitiators includes a light source constituted by a semiconductor base solid state radiation emitter which emits in the blue spectral range. Since the radiation emitter emits in a relatively limited spectral range excess heat radiation is avoided. The overall device is formed as a relatively small lightweight device with a built in battery. The device further includes a light-conducting rod in order to direct radiation to the desired location. According to the ""005 patent an essential photoinitiator in dental materials is typically camphorquinone or phosphine oxide which absorbs a broad band within the blue spectral range, with an absorption maximum of about 472 nm and 430 nm, respectively. The patent further notes that depending on the color of the material, the polymerization reaction requires light having an intensity of at least 1 to 5 mW/cm2 within a very thin layer. In the practice of polymerizing tooth fillings or dental replacement parts, a light intensity of at least 250 mW/cm2 is required within an appropriate period of time to achieve polymerization of sufficient degree and depth. Commercially available dental polymerization apparatuses, at least according to this ""005 patent, emit light at an intensity of about 400-500 mW/cm2 sometimes up to 700 mW/cm2. The solid state radiation emitter according to the ""005 patent is preferably a laser diode which emits a forward beam used for the polymerization proper and a backward beam used as a reference beam for controlling the intensity of the polymerization beam.
WIPO Publication No. WO 99/35995 (essentially same as U.S. Pat. No. 6,200,134) of Kovac et al. discloses a curing device for curing light sensitive compounds. The device includes generally a housing and an array of solid state light emitting diodes for having wavelengths in the range of 400-500 nm. Preferably, a peak wavelength of 470 nm is generated. The device further comprises an optical fiber light pipe for capturing the light and transmitting a beam of the light to the dental or other work surface containing a light curable compound. An optical lens may be used for focusing the light into the light pipe. It is noted on page 8 of this publication that 200-500 LEDs are used for creating the necessary light power needed for curing available dental compounds. In one embodiment of the device described, 96 LEDs are used whereas in a prototype e.g., an embodiment was made wherein 9 LEDs were utilized. See page 14. It is further noted in the publication that LEDs which include integral lenses may be employed. It should be noted that the LEDs according to this publication are generally arranged in a planar array. See, e.g., FIG. 3A. The discussion on page 20 and following notes that radiated power levels of approximately 200 mW/cm2 or greater are generally necessary for curing the available dental compounds. Other intensities may be necessary for curing other light sensitive compounds. Thus the description in the WIPO publication is generally directed to fairly high power levels.
U.S. Pat. No. 5,885,082 to Levy discloses the use of laser radiation having a selected wavelength and being in the form of pulses for cutting bone and performing dental procedures. There is disclosed in Column 4, lines 27 and following a filling material for teeth constituted by a mixture formed from a liquid component composed of phosphoric acid and water and a powder component composed of a ceramic and hydroxyapatite, with the ingredients mixed in a proportion to form a paste having a consistency such that the paste is workable and sufficiently self supporting to be applied to the opening with a spatula and remain in place. The ""082 patent does not involve a photocuring process and the material is not a dental polymer composite. The high peak power of the laser is believed only used for cutting and possibly hardening of the cement due to heat.
Very generally, there is provided in accordance with the present invention a device for curing dental compositions and curing methods that comprise exposing the dental composition to be hardened to radiation from an axial array of light emitting diodes (xe2x80x9cLEDsxe2x80x9d) having output wavelengths selected to photo-activate a hardening chemical reaction within the target composition.
The inventors have surprisingly discovered that relatively low power radiation from LEDs provides the same depth of cure as achieved by a conventional blue-light filtered dental gun, even though the LED irradiation intensity is between about 50% to 80% lower for the same exposure time. In particular, to achieve a 3.0 mm (1.5 mm ISO) depth of cure with a 60 second exposure time, an energy density of about 25 mW/cm2 at the target composition is required for an LED-based dental gun vs. an energy density of about 53 mW/cm2 required for a conventional blue-light dental gun. Remarkably, in the instance of a 4.0 mm (2 mm ISO) depth of cure with a 60 second exposure, an irradiation intensity of about only 38 mW/cm2 at the target composition is required for a single LED-based dental gun vs. about 200 mW/cm2 required for a conventional blue-light dental gun. Here the depth of cure is reported as the height of the cured cylinder. The ISO method reports the depth of cure as half the height of the cured cylinder and is sometimes given in parentheses as above and below.
Even more unexpectedly, it has been discovered that the amount of shrinkage that occurs during the curing process for irradiation intensities yielding a 3.0 mm (1.5 mm ISO) depth of cure in 60 seconds is about 7% lower when a single-LED-based dental gun is employed instead of a conventional blue light dental gun. In addition, the smaller size of an LED permits a smaller dental gun to be employed so that the level of discomfort experienced by a patient is decreased. Even further, for irradiation intensities yielding a 3.0 mm (1.5 mm ISO) depth of cure in 60 seconds, the degree of heating has been measured and found to be about 9% less when the LED-based dental gun of the present invention is employed instead of a conventional blue light dental gun. Thus the use of the present invention causes a lower thermal discomfort to the patient.
For depths of cure higher than 4.0 mm (2 mm ISO) the inventors have find surprisingly good curing characteristics with an LED gun using four LEDs. In particular, to achieve a 4.5 mm (2.25 mm ISO) depth of cure under a 40 second exposure, an energy density of about 180 mW/cm2 at the target composition is required for the four-LED gun vs. an energy density of about 450 mW/cm2 required for a conventional blue-light dental gun. Remarkably, for these intensities, a 40 second exposure results in a temperature rise that is up to 50% lower in the case of the four LED device as compared to the conventional blue-light dental gun. The amount of shrinkage is about 10% lower than that resulting from curing using a conventional dental and the top and bottom surface hardness obtained using the four-LED device is at least as good as that obtained with a conventional lamp.
Exemplary LEDs useful in practicing the present invention include Panasonic""s xe2x80x9cLED Blue Clearxe2x80x9d 1500 millicandela T1-xc2xe, LNG992CFBW and similar devices commercially available from Hewlett Packard, Toshiba and Nichia. Such LEDs emit radiation in the range from about 440 to about 500 nanometers with a power output of about 1500 millicandela. A programmable power supply 24 employed in conjunction with the above identified Panasonic LED is well know in the industry; specifically a model PS 281 produced by Tektronix may be used to obtain the results described below. It has also been discovered that LED""s may be operated relatively close or at their maximum luminous output. For the Nichia LEDs with model number NSPB500S-XF3 the maximum continuous current specification is 30 milliamperes with recommended value at 20 milliamperes. Pulsed operation with a maximum current at 100 milliamperes is recommended only for a maximum duty cycle of 10%. At these operating conditions LEDs typically have lifetimes of over 50 thousand hours. However in our tests, a Nichia NSPB500S-XF3 was kept xe2x80x9conxe2x80x9d for about 10 days continuously at a current of 90 milliamperes with no deterioration in its output. For dental curing applications, the LED stays on only for about 60 seconds at a time and therefore this higher current is not expected to alter the LED output characteristics for a lifetime that is comparable to the typical lifetime at recommended operating conditions.
Specific exemplary compositions may include: TPH Spectrum composite (shade A3.5) from Dentsply International, Inc. wherein the resin matrix of the composite consists of a BisGMA-adduct (adduct of 2,2-Bis[4-2-hydroxy-3-methacryloyloxpropoxy)-phenyl]propane with hexamethylene diisocyanate), ethoxylated Bisphenol-A-dimethacrylate (Bis-EMA, 2,2-Bis[4-(2-methacryloyloxyethoxy)-phenyl]propane) and triethylene glycol dimethacrylate. The combination of barium alumino boro silicate glass filler with a mean particle size below 1 xcexcm and colloidal silica (particle size of about 0.04 xcexcm) results in a hybrid composite with good strength and wear resistance for posterior use, combined with high surface luster and smoothness, which is an essential property for anterior use of a composite; as well as adhesives including light cure resin bond from Reliance which includes Bis-GMA and ethoxylated derivative, Polyethylene glycol dimethacrylate amine, Ketone photoinitiator. Filler particles include 60-99% fused silica and 7-13% amorphous silica. U.S. Pat. No. 5,711,665 suggests the use of a single-LED for bonding of orthodontic brackets to the tooth surface. However the use of a single LED here is to provide a xe2x80x98tackxe2x80x99 bond that temporarily secures the orthodontic bracket to the tooth. xe2x80x9cSubsequently, remaining portions of the adhesive between the bracket base and the tooth are cured by another curing light assembly, possibly emitting a greater intensity of light can be used.xe2x80x9d This indicates that the single LED curing in this patent is intended more for temporarily forming a xe2x80x9ctackxe2x80x9d bond rather than completely curing the adhesive. Therefore complete curing of adhesives is also within the scope of the present invention. Surprisingly our tests with the four LED device yields a depth of cure of several mm (typically 4 mm (2 mm ISO) or more) in the adhesives with a 40-60 second exposure, far more than the cure depth that is typically required for the adhesive layer.
There is provided in accordance with the present invention a device for curing a photosensitive dental composition curable by way of irradiation with light of predetermined wavelength including as components:
(a) a power supply; and
(b) a radiation source coupled to the power supply and powered thereby, the radiation source having a radiation output and including an axial array of light emitting diodes, wherein the axial array of light emitting diodes has a plurality of light emitting diodes mounted about an assembly axis defined thereby at a plurality of axial distances from the radiation output of the radiation source and being further characterized in that the plurality of light emitting diodes are directed along a plurality of divergent paths such that a position insensitive optical field is generated for curing the dental composition In a preferred device, the plurality of light emitting diodes comprises 4 light emitting diodes with a first pair of light emitting diodes mounted a first distance from the radiation output of the radiation source and a second pair of light emitting diodes mounted a second distance from the radiation output of the radiation source, the second distance being greater than said first distance. The second distance may be greater than said first distance by from about 0.5 mm to about 2 mm; typically the second distance is greater than the first distance by about 1 mm.
Each of said light emitting diodes is usually mounted about the assembly axis at an angle of from about 10xc2x0 to about 30xc2x0; typically at an angle of about 20xc2x0. The device may include a lens, the outer surface of which lens defines the radiation output of the radiation source. The lens may be an aspherical lens. So, also, each of the light emitting diodes may have a characteristic maximum luminous power output and the power supply may be adapted to operate each of said light emitting diodes at a luminous power output of at least about 85 percent of its maximum luminous power output.
The position insensitive optical field is generally uniform over an axial distance of from about 1 to about 5 mm from said radiation output of said radiation source.
In another aspect of the present invention there is provided a method of curing a photosensitive dental composition curable by way of irradiation with light of predetermined wavelength by way of:
A. applying the said photosensitive dental composition to a dental surface, or a cavity; and
B. irradiating the dental composition with a position insensitive optical field generated by way of an irradiation source including an axial array of light emitting diodes, wherein the axial array of light emitting diodes comprises a plurality of light emitting diodes mounted about an assembly axis defined thereby at a plurality of axial distances from the radiation output of the radiation source and being further characterized in that the plurality of light emitting diodes are directed along a plurality of divergent paths.
Still further aspects and advantages of the present invention will become readily apparent from the discussion which follows.